The present invention relates to methods of epitaxially growing compound crystals and doping methods therein and, more particularly, to a method of epitaxially growing a compound crystal, having controllability of film thickness of the monomolecular order as a technique for growing a thin-film crystal of a semiconductor and to a doping method in epitaxial growth of a compound crystal having strict controllability of dopant impurities in addition to the controllability of the film thickness of the monomolecular order.
Conventionally, molecular beam epitaxy (hereinafter referred to as "MBE"), metal organic-chemical vapor deposition (hereinafter referred to as "MO-CVD") and the like are generally employed as a technique for growth of a thin-film crystal for a semiconductor, for example. Particularly, in the case of a compound semiconductor, these methods have widely been used in manufacturing of a semiconductor device.
The aforesaid MO-CVD has widely been utilized for the reason that an apparatus for the MO-CVD can be used in which its structure or construction is relatively simple and easy and which is low in cost, for the reason that productivity is superior in which growing speed or rate is high and in which growing time is short, and for other reasons. However, it has been impossible for the MO-CVD to control growing film thickness of the order of a monomolecular layer.
The doping technique due to the MO-CVD has been arranged such that compound gas of an element for forming a semiconductor and compound gas of dopant are simultaneously directed onto a substrate crystal.
Further, the aforementioned MBE uses a method in which a raw material is heated and vaporized, and the vapor is evaporated or deposited onto the substrate crystal while controlling a molecular beam. A crystal film is extremely thin, and it is possible to sufficiently control composition, profile and crystal growing rate. Accordingly, the MBE is superior in controllability of the growing film thickness as compared with the aforesaid MO-CVD.
The doping method in the MBE is arranged such that vapor of a dopant element is vaporized onto the substrate crystal, simultaneously together with vapor of a constitutional element which serves as a raw material.
In order to produce a crystal superior in quality by the MBE, it is required for the case of, for example, GaAs to set the crystal growing temperature to high temperature of 500.degree. to 600.degree. C. It will become a problem that, because the temperature is high, in the case where steep or sharp impurity profile such as npn and pnp is produced, the impurities are redistributed. Further, since the MBE is based on an evaporation method, there are a problem of deviation from stoichiometry composition of a growing film, and a problem of high density of surface defects such as oval defect or the like.
Because of these defects, in recent years, attention has been paid to molecular-layer epitaxy (hereinafter referred to as "MLE") having controllability of film thickness of the order of a monomolecular layer.
The MLE is a method in which, in the case of crystal growth of III-group through V-group compounds, III-group compound gas and V-group compound gas are alternately directed onto a substrate crystal, and the crystal is grown a monomolecular layer by a monomolecular layer.
This technique is reported in a paper [J. Nishizawa, H. Abe and T. Kurabayashi; J. Electrochem. Soc. 132 (1985) 1197-1200] written by Jun-ichi Nishizawa et al, for example.
The aforesaid MLE utilizes adsorption and surface reaction of the compound gasses and, in the case of, for example, III- through V-group crystals, produces a monomolecular-film growing layer by introduction of the III-group compound gas and the V-group compound gas one by one.
In the manner described above, since the MLE utilizes monomolecular-layer adsorption of the compound gasses, growth can be done always of the order of the monomolecular layer within a certain pressure range even if pressures of the introducing gasses varies.
Furthermore, the MLE utilizes trimethylgallium (TMG) that is alkylgallium and arsine (AsH.sub.3) that is a hydrogen compound of arsenic. However, in place of the above-mentioned TMG, the MLE uses triethylgallium (TEG) that is alkylgallium, whereby it is possible to produce a GaAs growing layer high in purity, by growth at further low temperature.
This technique is reported in a paper [J. Nishizawa, H. Abe, T. Kurabayashi and N. Sakurai; J. Vac. Sci. Technol. A4(3), (1986) 706-719] written by Jun-ichi Nishizawa et al, for example.
Moreover, the doping method of the GaAs due to the MLE is carried into practice such that, in addition to the compound gasses of the TEG and AsH.sub.3, the compound gas of dopant is successively directed onto a surface of the substrate crystal separately. By doing so, it is possible to produce a crystal high in purity, which is controlled in impurity, including a high-dope crystal, at low temperature.
Furthermore, since the MLE grows the crystal film under low temperature, redistribution of impurities is extremely low, so that it is possible to realize steep or sharp impurity profile.
This technique is reported in a paper ]J. Nishizawa, H. Abe and T. Kurabayashi; J. Electrochem. Soc. Vol. 136, No. 2, pp. 478-484 (1989)] written by Jun-ichi Nishizawa et al, for example.
By the way, in order to produce, for example, a high-speed semiconductor device, it will be required to produce a growing film of impurity density further high in concentration, and to realize crystal growing at further low temperature.
Accordingly, it is desirable to further improve the doping method in the epitaxial growth of the molecular layer.